The SV40 small t antigen (t) provides a helper function which enhances the apparent neoplastic transforming activity of the major SV40 transforming protein, large T antigen (T). The mechanism underlying the helper function isn't known. This application proposes an effort designed to gain insight into this question. Specifically, we will undertake a genetic analysis of the t structure-function relationships underlying three t functions - the helper effect and two newly recognized t activities, the ability to transactivate selected pol II and III promoters and to bind the catalytic and regulatory subunits of protein phosphatase 2A. One major goal of these experiments is to know whether either or both of these biochemical activities underlies the helper effect. In addition, an effort will be made to understand the biochemical mechanisms employed by t in the transactivation process. This information could also prove useful in future efforts designed to understand how this protein functions in the viral transforming process.